It is already known to use light guides to illuminate panels for general lighting purposes and for display applications (e.g. for illuminating signs and advertisements, and also for illuminating liquid crystal displays). In one form, often referred to as a light box, the light guide comprises a hollow box-shaped structure defining an optical cavity, and in another form it comprises a solid light-guiding plate. In both forms, a major surface of the guide can be illuminated by light directed into the guide in a direction generally parallel to that major surface, for example from at least one elongated light source located adjacent an edge of the light guide.
Light guides in the form of hollow light boxes are described, for example, in EP-A-0 490 279; 0 377 309; and 0 293 182; and in GB-A-2 310 525. In each of those light boxes, a prismatic optical film is employed with a view to achieving a more even distribution of light over the surface that is being illuminated. In addition, an Application Bulletin entitled “Thin Light Box” and issued in March 1990 by Minnesota Mining and Manufacturing Company of St. Paul, Minn., USA describes the design and construction of light boxes, using Scotch™ Optical Lighting Film in combination with a shaped sheet of V-5115 Scotch™ Light Extractor Film, for use in illuminating graphic displays.
In the case of light guides in the form of solid light-guiding plates, it is well known to form light-extraction elements of some type on the rear major surface of the plate with a view to achieving a more even distribution of light over the front surface (i.e. the surface that is being illuminated). In some cases, printed light-extraction elements are used and are applied directly to the rear surface of the light-guiding plate. Arrangements of that type are described, for example, in U.S. Pat. Nos. 5,736,686; 5,649,754; 5,600,462; 5,377,084; 5,363,294; 5,289,351; 5,262,928; 5,667,289; and 3,241,256.
U.S. Pat. No. 5,618,096 describes light-emitting panels of various types and mentions the possibility of providing light-extracting deformities on one or both sides of a panel to control the amount of light emitted from any area of the panel. It is also mentioned that the deformities may be printed on a sheet or film which is used to apply the deformities to the panel member. WO 92/05535 describes an illuminated display system comprising a transparent panel with a dot matrix applied to both sides and an opaque backing sheet attached to the rear side. An image to be illuminated is printed on a further sheet positioned on the front side of the panel.
As recognised in previous disclosures, the problems to be addressed in constructing a light guide for illumination purposes include achieving a uniform level of brightness across the panel that is being illuminated, and minimising the amount of power required to produce a level of illumination that is adequate having regard to the circumstances. As regards the first of those problems, it is often the case that the panel is more brightly illuminated in the area closest to the light source, which detracts from the overall visual appearance and effectiveness of the illumination. As regards the second of those problems, it is clearly highly desirable, from an environmental and a cost point of view, that the amount of power used for illumination purposes should be kept as low as possible. Moreover, when the power is derived from a battery (as may be the case when LCD and computer displays are being illuminated) it is also generally desirable that the amount of power utilized should be minimized so that the battery can be kept as small and light as possible.
In addition to those considerations, it would be advantageous to be able to produce, comparatively easily and in a cost-effective manner, light guides of widely-differing dimensions that would be suitable for use in a variety of situations but would, nevertheless, function with a high level of efficiency.